ABSORPTION SPECTROPHOTOMETRY 



147 



32 



,28 



£24 



20 



= 16 



2200 



2400 2600 



Wavelength A 



2800 



Fig. IV-9. The ultraviolet absorp- 



showing the comparable transmis- 

 sivity of human sweat about 1 mm 

 thick (Crew and Whittle [1938]). 



The lower series of graphs illus- 

 trates how the extinction coefficient 

 k can be used to show the relative 

 absorption of (3) egg albumin, (4) 

 serum pseudoglobulin, and (5) serum 

 albumin. Note that they all have « 

 a pronounced absorption band at =12 

 2790 A, which is common to proteins, 

 and are rather transparent to radia- 



o 



tion around 2500 A. 



Figure IV-9 illustrates the use of 

 the molecular extinction coefficient 

 to show that the combined absorp- 

 tion of several constituent molecular 



, ,1 1 ,• tion spectrum of thymus nucleic acid 



groups is equal to the absorption , ..,., , 



^ M ^ as compared with the absorption spec- 



of the composite molecule. The tra of its purine and pyr i m i d i ne con- 

 broken graph shows how closely the stituents. The graphical sum of the 

 sums of the extinction coefficients absorbing constituents is the upper 



of the products of hydrolysis of broken . curve - Note how closel y [t 

 ., , . • i r • / • approximates the experimental value. 



thymus nucleic acid [purine (guanine (% courtegy of j R Loofbourow 



and adenine), and pyrimidine con- 1940].) 



stituents (thymine and cytosine)] 



resemble the experimental values obtained from the composite molecule, 



thymus nucleic acid. 



Absorption Spectrophotometry 



The essential pieces of apparatus for absorption spectrophotometry 

 are: (1) a constant source of radiation, (2) an optical instrument for 

 resolving the radiation into a spectrum, and (3) a means of evaluating 

 the relative intensities of the incident and transmitted energy passing 

 through an absorption cell. 



When absorption measurements in the visible spectral regions are made, 

 incandescent linear-filament lamps are used. For the ultraviolet, the 

 condensed under-water spark between molybdenum terminals gives a 

 nearly continuous strong spectrum containing wavelengths as low as 

 2000 A. Figure IV-20 shows the limits of transmission set by very 

 pure water. Recently developed high-pressure mercury arcs (Buttolph 

 [1939]) emit a practically continuous spectrum in the long-wave ultra- 

 violet. 



